A brazing method has been widely used as a method for producing an aluminum product (e.g., aluminum heat exchanger or machine part) having a large number of small joints. When brazing aluminum (including an aluminum alloy), it is indispensable to break the oxide film that covers the surface of the material so that the molten filler metal comes in contact with the matrix or another molten filler metal. The oxide film can be broken by utilizing a method that utilizes a flux, or a vacuum heating method, for example. These methods have been put to practical use.
The brazing method is applied to various fields. The brazing method is most typically applied to automotive heat exchangers. Most of the automotive heat exchangers (e.g., radiator, heater, condenser, and evaporator) are made of aluminum, and produced by applying the brazing method. A method that applies a non-corrosive flux to the material, followed by heating in a nitrogen gas atmosphere is most widely used at present.
In recent years, a heat exchanger provided with electronic parts (e.g., inverter cooler) has been used along with a change in driveline (e.g., electric car and hybrid car), and a flux residue has increasingly posed problems. Therefore, some of the inverter coolers are produced using a vacuum brazing method that does not utilize flux. However, since the vacuum brazing method utilizes a heating furnace that increases the equipment cost and the maintenance cost, and has problems as to productivity and brazing stability, a brazing method that is implemented in a nitrogen gas furnace without using flux has been increasingly desired.
In order to deal with the above demands, the inventors proposed a clad material that is used to effect brazing in an inert gas atmosphere without using a flux, and is produced by heating a core material and a filler metal in a state in which a metal powder is interposed between the core material and the filler metal, to a temperature equal to or higher than the solidus temperature of the metal powder to produce a liquid phase in the metal powder and bond the core material and the filler metal, and subjecting the core material and the filler metal to hot clad rolling, the metal powder including at least one of Li, Be, Ba, Ca, and the like, and having a solidus temperature lower than the solidus temperature of the core material and the solidus temperature of the filler metal.
When the above clad material is used, an oxide is not formed on the surface of the filler metal during material production, differing from a case where Li, Be, Ba, Ca, and the like are added to the filler metal. Since Li, Be, Ba, Ca, and the like are dissolved and diffused in the molten filler metal during brazing, and the oxide film formed on the surface of the molten filler metal is weakened, brazability can be effectively improved.
However, the method that supplies Li, Be, Ba, Ca, and the like to the filler metal using a metal powder has the following problems in terms of material production. Specifically, when a clad material is produced in a plant, it is necessary to provide a large amount of metal powder between the core material and the filler metal since the filler metal that has not been rolled has a considerable thickness. When Li, Be, Ba, Ca, and the like are added in an increased amount, a strong oxide film is formed on the surface of the metal powder. Since the oxide film does not break even when heated to a temperature equal to or higher than the solidus temperature of the metal powder, it is difficult to uniformly bond the core material and the filler metal. If the metal powder remains at the boundary between the core material and the filler metal in the form of a powder, the cladding capability during hot rolling is affected, whereby peeling may occur during rolling, or blistering may occur during softening (heating). Since a large amount of a metal powder that has a strong oxidizing capability is used, it is necessary to take special safety measures in the production site. It is also necessary to strictly prevent the metal powder from being mixed with another material. Therefore, it is difficult to produce a product having stable quality, and an increase in cost occurs.
A method that implements brazing in an inert gas atmosphere without using a flux by diffusing Mg into the filler metal during brazing has been proposed. For example, a method that diffuses Mg added to the core material into the filler metal, and a method that diffuses Mg added to a sacrificial anode material provided between the core material and the filler metal into the filler metal, are known. These methods may prevent a situation in which an oxide film is formed on the surface of the filler metal during production of the clad material or during brazing, and it may be considered that Mg is effective for breaking the oxide film formed on the surface of the filler metal.
However, the functions of the core material and the sacrificial anode material included in the clad material may be impaired by the addition of Mg. Specifically, when the Mg content is increased, erosion may occur to a large extent due to molten filler metal, or corrosion resistance may be adversely affected. On the other hand, when the Mg content is limited, it may be difficult to sufficiently break the oxide film formed on the surface of the filler metal. When Li, Be, Ba, and Ca are added to the core material or the sacrificial anode material, it is difficult to break the oxide film since the amount of addition is further limited.